Field of the Invention
The present invention relates to a technique of improving a diaphragm type fuel pump for a general purpose engine.
Description of the Related Art
A fuel pump used in a fuel supply line for an engine mounted in a vehicle is made up of an electrical pump such as a solenoid pump. However, in most cases, the general purpose engine does not have any battery. Therefore, the fuel pump used in such a general purpose engine cannot receive supply of electrical energy at the time of starting operation of the general purpose engine. The fuel pump needs to be a mechanical pump for making it possible to supply a fuel at the time of starting operation of the engine.
The mechanical pump supplies the fuel to the general purpose engine from a fuel tank provided outside the general purpose engine. The mechanical pump is driven by a camshaft of the general purpose engine. For example, the mechanical pump is a diaphragm type fuel pump. For example, a diaphragm type fuel pump disclosed in Japanese Patent No. 4563613 (Patent Document 1) is known.
The diaphragm type fuel pump for the general purpose engine includes a housing, a diaphragm, a rod, a plunger, and a plunger biasing member. The diaphragm divides a space inside the housing into a pump chamber and an air chamber. One end of the rod is coupled to the diaphragm, and the other end of the rod protrudes from the air chamber to the outside of the housing. The rod is slidably supported by the housing. The plunger is positioned concentrically with the other end of the rod, and slidably fitted with respect to the rod. Further, the plunger biasing member biases the plunger toward the other end of the rod.
The other end of the rod includes a pin extending in a radially outward direction of the other end. The plunger includes a bottomed tubular shape having a tubular portion. The other end of the rod can be fitted into the tubular portion. A long hole extends through the tubular portion in a radial direction. The long hole is elongated in the longitudinal direction of the plunger. A pin contacts one part of the edge of the long hole closer to the diaphragm.
The discharge pressure of the fuel pump is a relatively low pressure which depends on the biasing force of the diaphragm biasing member. The diaphragm biasing member biases the diaphragm from the air chamber to the pump chamber.
In this regard, even in the case of a general purpose engine which does not have any battery, it may be preferable to provide an electronic fuel injection apparatus. Since the electronic fuel injection apparatus injects a fuel at high pressure, in comparison with the case of a carburetor which atomizes the fuel utilizing the negative pressure, the state of atomization can be controlled easily. The discharge pressure of the fuel pump for supplying the fuel to the electronic fuel injection apparatus needs to be a high pressure.
However, the discharge pressure of the diaphragm type fuel pump for the general purpose engine disclosed in the above Patent Document 1 is a low pressure. It is necessary to increase the biasing force of the diaphragm biasing member for allowing the discharge pressure to become the high pressure. However, if the biasing force of the diaphragm biasing member becomes excessively high, the speed of the elastic displacement of the diaphragm cannot follow the displacement speed of a cam nose provided in a camshaft of the general purpose engine. Therefore, there is a limit on increase in the discharge pressure of the diaphragm type fuel pump for the general purpose engine. There is a room of improvement in performing atomization of the fuel properly by the electronic fuel injection apparatus.
In this regard, in the diaphragm type fuel pump for the general purpose engine, it may be considered to directly connect the other end of the rod to the plunger, or form the plunger integrally with the other end of the rod. When the plunger is pushed up by the cam of the general purpose engine, the diaphragm is forcibly displaced elastically toward the pump chamber by the plunger. Therefore, the resulting discharge pressure of the fuel pump becomes relatively high.
However, in the case of simply adopting a structure where the plunger is directly connected to the other end of the rod, or the plunger is formed integrally with the other end of the rod, the structure becomes complicated. Further, the cost of the diaphragm type fuel pump becomes high. Further, the diaphragm type fuel pump for the general purpose engine to be used in the low pressure application and the diaphragm type fuel pump for the general purpose engine to be used in the high pressure application need to be produced individually. Consequently, the diaphragm type fuel pump for the general purpose engine to be used in the low pressure application and the diaphragm type fuel pump for the general purpose engine to be used in the high pressure application may have different shapes and sizes. There is a room of further improvement in this regard.